In recent years, methods of producing a sugar liquid by pretreating a cellulose-containing biomass with an acid, hot water, alkali or the like and then adding cellulase thereto to perform hydrolysis have been widely studied. However, these methods using cellulase have a drawback in that, since a large amount of cellulase is used and cellulase is expensive, the cost of producing a sugar liquid is high.
As methods of solving the problem, methods wherein the cellulase used for hydrolysis of cellulose is recovered and reused have been proposed. Disclosed examples of such methods include a method wherein continuous solid-liquid separation is carried out with a spin filter and the obtained sugar liquid is filtered through an ultrafiltration membrane to recover cellulase (JP 2006-087319 A), a method wherein a surfactant is fed at the stage of enzymatic saccharification to suppress cellulase adsorption and thereby enhance the recovery efficiency (JP 63-087994 A) and a method wherein the residue produced by enzymatic saccharification is subjected to electric treatment to recover the cellulase component (JP 2008-206484 A), but these methods failed to fundamentally solve the problem.
There is thus a need to reduce the amount of cellulase used in hydrolysis of cellulose.